The present invention relates to interference suppression systems and more particularly to side-lobe canceller systems which are compatible with MTI.
Generally, interference suppression systems of the prior art are designed to reduce the presence of undesired signals in a signal receiving system. As is known, in particular systems, such as a radar system, the characteristics of the receiving antennas are such that undesired signals which are received in the side-lobes interfere with the isolation of the target signal received in the main lobe. Accordingly, to isolate the main lobe signals, side-lobe cancellers have been used to cancel interference in the side-lobe of the main radar antenna as exampled by the Paul W. Howells U.S. Pat. No. 3,202,990.
It has been found that while conventional side-lobe canceller systems are highly successful in most instances, problems arise when the system is used to cancel interference in a high clutter environment where normally Moving Target Indicators (MTI) can be used to cancel the clutter. An MTI relies on the clutter return being the same on a pulse to pulse basis in order to allow distinction from a target return which varies due to target movement between radar pulses. When clutter is present simultaneously with jamming interference, however, it has been found that conventional side-lobe cancellers modify the clutter on a pulse to pulse basis because of a cross modulation between the clutter and interference signals. As the clutter returns are not the same, an MTI cannot be used thereby preventing clutter cancellation and prohibiting the use of a side-lobe canceller with the MTI.
It has been proposed that one way to avoid clutter modification with side-lobe cancellers is to sample the interference signal just prior to the transmission of a radar pulse when clutter is weak or non-existent. While in theory such sampling would allow the derivation of side-lobe cancelling signals without clutter modification, it would also introduce a time dependent unbalance in the cancelling signals due to the antenna scan between sampling pulses. In practice, systems for implementing such sampling and cancelling still remain undeveloped with the result being that there is presently no available system for allowing effective and compatible use of a side-lobe canceller system with an MTI.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a system for producing reliable interference cancellation in a high clutter environment.